1. Field of the Invention
Example embodiments of the present invention relate to equalizers, which may be used in communication systems, and methods for the same.
2. Description of the Conventional Art
An equalizer may a signal processor, which may be used in various types of signal transmission and receipt systems (e.g., communications, broadcasting, data storage, and/or for military purposes) and may be used to compensate for distortions of signals input to, or output from, various types of signal transmission and receipt systems. A filtering circuit, which may be an element of an equalizer, may improve the performance of a communications system, for example, by suppressing channel noise and/or distortions caused by the delay of signals input to, or output from, the communications system via one or more paths. The filtering circuit may use filtering coefficients in order to suppress channel noise and/or distortions. The values of the filtering coefficients may be determined based on channel estimation information and/or noise signals distributed over upper and/or lower frequencies of main data signals (e.g., delayed signals). The filtering coefficients corresponding to noise signals may be set to values such that they may be used for suppressing the respective noise signals.
FIG. 1 is a block diagram of a conventional equalizer 10. Referring to FIG. 1, the conventional equalizer 10 may include a filter circuit 11, a slicer 12, a coefficient generator 13, and an operation circuit 14. The filter circuit 11 may include a feedforward filter 31, a feedback filter 32, and an adder 33. The feedforward filter 31 may include filter taps F1 through FG (where G is an integer) and an adder 51. The feedback filter 32 may include filter taps P1 through PG (where G is an integer) and an adder 52. The filter taps F2 through FG and P1 through PG may include a data buffer 41, a coefficient circuit 42, and a multiplier 43. The coefficient circuit 42 may include a multiplier 44 and a coefficient buffer 45. The filter tap F1 may not include a data buffer. The operation circuit 13 may include an error calculation circuit 21 and a multiplier 22.
The coefficient generation circuit 14 may estimate channel variations based on an input data signal Din and may generate filtering coefficients Co1 through Co(2G) (where G is an integer) based on the estimated channel variations. The input data signal Din may include main data signals and noise signals. The noise signals may be delayed main data signals, which may be generated when transmitting the main data signals along multiple paths. The coefficient generation circuit 14 may generate filtering coefficients Co1 through Co(2G), which may offset the noise signals in order to suppress the noise signals. The filter taps F1 through FG of the feedforward filter 31 and the filter taps P1 through PG of the feedback filter 32 may suppress the noise signals included in the input data signal Din, for example, using the filtering coefficients Co1 through Co(2G).
A less precise channel estimation may cause abnormal filtering coefficients whose values may not be equal to zero but may be substantially close to zero to be generated in coefficient buffers of filter taps where the main data signals or the noise signals may be rarely located. Abnormal filtering coefficients may cause distortion of an output data signal Dout and/or may lower the convergence speed of the filter circuit 11. Abnormal filtering coefficients may have smaller values than normal filtering coefficients. In order to reduce the likelihood that the output data signal Dout may be distorted, conventional equalizers may indiscriminately set filtering coefficients having smaller values than a threshold value to zero. However, the channel estimation information may still be imprecise, and the location and/or magnitude of the main data signals and the noise signals may differ from the channel estimation information. On time-varying multi-paths along, which signals may variably propagate, the location and/or magnitude of the main data signals and the noise signals may often vary. For example, a main data signal may move from the filter tap F2 to the filter tap F4, and the magnitude of the main data signal may increase or decrease. The main data signals or the noise signals may move from one filter tap to another filter tap. If filtering coefficients having smaller values than a threshold value are indiscriminately set to zero, filtering coefficients of filter taps to which the main data signals may be expected to move may be falsely set to zero as well. This may considerably deteriorate filtering results obtained by the filter circuit 11.
The filtering coefficients stored in the coefficient buffers 45 of the filter taps F1 through FG and P1 through PG may be updated based on operation data S. The operation data S may be obtained by the operation circuit 13 and symbol data (not shown) of the input data signal, and the coefficient buffers 45 of the filter taps F1 through FG and P1 through PG may store the updated filtering coefficients. For example, if CN corresponds to a current filtering coefficient and CN+1 corresponds to a subsequent filtering coefficient to be updated, the subsequent filtering coefficient CN+1 may be expressed by Equation (1):CN+1=CN+μE·dx  (1)
where μ is a step size coefficient, E is error data, dx is one of the symbol data of the input data signal Di, and μE is the same, or substantially the same, as the operation data S. The subsequent filtering coefficient CN+1 may be proportional to the step size coefficient μ. As the step size coefficient μ increases, the speed of the filter circuit 11 tracking channel variations may increase, and the number of errors remained in a signal filtered by the filter circuit 11 may decrease. The step size coefficient μ may be inversely proportional to the number of filter taps currently operating in the filter circuit 11. That is, as the number of filter taps operating in the filter circuit 11 increases, the step size coefficient μ may decrease. Alternatively, the smaller the number of filter taps operates in the filter circuit 11, the larger the step size coefficient μ. If all, or substantially all, of the filter taps F1 through FG and P1 through PG, in the conventional equalizer of FIG. 1, operate all, or substantially all, the time, there may be a limit in the size of the step size coefficient μ. If the number of filter taps included in the filter circuit 11 is reduced to increase the step size coefficient μ, the filter circuit 11 may not filter noise signals input thereto after an elongated delay period, which may considerably deteriorate the filtering performance of the filter circuit 11.